Mercury is the closest planet to the sun. It is the densest planet in our solar system. It is the planet that we have looked at the least. It has been explored less than any of the four inner planets.These are some of the reasons NASA is sending a mission to Mercury. MESSENGER was launched in the summer of 2004. The name stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging. It is NASA's first mission to Mercury in 30 years. It will be the closest look ever at the innermost planet in our solar system. MESSENGER will study Mercury in three flybys. Then it will settle into orbit in 2011. It will send data back to Earth for a year. MESSENGER will map the surface of Mercury. It will send back data about Mercury's thin atmosphere. It will also send back data on the magnetic fields and chemical make-up. Scientists hope to learn why Mercury is so dense. As it flies by, MESSENGER will observe Earth. Then on its way to Mercury, it will also observe Venus. MESSENGER will be in very high temperatures during the mission. This is because it will be so close to the sun. The surface of Mercury can reach 840 degrees Fahrenheit. The instruments on MESSENGER are very delicate. They are behind an insulated sun shade. They will stay a cool 80 degrees. The instruments will stay behind the shade while they study the planet. The data they gather will tell us more about the formation of the solar system.